Vascular cognitive impairment (VCI) is an insidious disease that progressively destroys memory and cognitive function with age. With a growing elderly population in the US, VCI will become a significant healthcare burden within the coming decades. Cerebral microinfarcts are small brain lesions (0.1 to 3 mm in diameter) that arise from obstruction of small cerebral vessels. They are estimated to be twice as prevalent in the demented brain. Recent advances have demonstrated the feasibility of detecting microinfarcts with 7T magnetic resonance imaging (MRI), supporting their potential as imaging biomarkers of VCI. However, a critical barrier for current progress is an inability to link these anomalous signals with microinfarcts of a specific age, size, and cellular pathology. Our long-term goal is t elucidate microinfarct-based tissue pathologies that generate signal contrast in MRI methods such as diffusion-weighted imaging (DWI), in order to optimize their detection non- invasively. Our approach is to longitudinally image microinfarcts in mice using in vivo two-photon laser- scanning microscopy (TPLSM), a method that can repeatedly probe cellular structure in the intact brain with micrometer precision. By combining TPLSM and MRI of the same animals, we will be able to link specific pathological events at the cellular level with respective DWI signals as they evolve over time. We will use a novel mouse model of microinfarction that we have developed, which provides exquisite control over lesion size, location, and timing of onset. The objective of this proposal, which is in pursuit of our long-term goal, is to determine how the reactivity of glial cells can influence DWI signal contrast. Our central hypothesis is that dramati tissue density change caused by microglial infiltration and astroglial proliferation during microinfarction is a principle source of DWI signal change. This hypothesis is formulated based on our past histological work, which revealed that rodent microinfarcts, like human microinfarcts, are densely packed with GFAP-positive astrocytes and CD68-positive macrophages in the sub-acute period of days to weeks. We will test the hypothesis with two Aims. In Aim 1, we plan to characterize the time-course of microglial and astroglial reactivity during microinfarct growth using TPLSM in two transgenic mouse lines that specifically express fluorescent proteins in those cells. In Aim 2, we will examine the correlation between microinfarct-specific DWI signal and the spatiotemporal change in glial reactivity measured by TPLSM from the same animals. Finally, we will test whether a new DWI technique, diffusional kurtosis imaging (DKI) can improve the sensitivity of microinfarct detection, compared to conventional DWI. Our approach is innovative because it uses a completely in vivo strategy to link cellular pathology with MRI signals during microinfarction by combining two powerful imaging modalities: TPLSM and 7T MRI. The proposed research is significant because the results can immediately impact the interpretation of small anomalous signals seen by DWI, and may therefore improve detection of VCI in aged individuals before the onset of cognitive impairment.